1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine blade with a spar and shell construction.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine is a very efficient machine to convert a high temperature gas flow into mechanical energy. An industrial gas turbine engine, fuel efficiency and performance are very high priorities. Also, long running times are very important since these engines typically run for 48,000 hours before shutdown. Thus, part life is also critical.
Higher engine efficiencies can be obtained by passing a higher temperature gas into the turbine inlet. However, the maximum temperature at turbine inlet is related to the material capabilities of the first stage airfoils which include the stator guide vanes or nozzles and the rotor blades.
Rotor blades and stator vanes can be made from a spar and shell construction in order to allow for exposure to higher gas flow temperatures because of the use of exotic high temperature metals. U.S. Pat. No. 7,080,971 issued to Wilson et al on Jul. 25, 2006 and entitled COOLED TURBINE SPAR AND SHELL BLADE CONSTRUCTION (incorporated herein by reference) shows this type of blade. A shell made from a very high temperature material (such as ODS, CMC and Molybdenum) can withstand higher temperatures than the present day nickel alloys. However, these high temperature materials cannot be cast or machined. A special process is required to form the shell such as wire EDM. Therefore, a turbine blade made from the spar and shell construction can be used to increase the allowable turbine inlet temperature in order to further increase the efficiency of the engine.
In the above described Wilson et al patent (U.S. Pat. No. 7,080,971) the shell is held in place between the underside of the spar tip edge and the root in the platform of the blade assembly. The tip portion of the shell will form a tight and effective butt seal against the retaining head on the underside of the spar tip edge due to centrifugal loading. However, the bottom end of the shell slides within a groove formed on a top surface of the platform. Hot gas ingestion can occur through this interface and into the interior of the spar and shell construction.